How to simplify or resample a Bezier curve in Blender 2.9?
Consider the following gif:


Here I have simply added a curve object, and then used Shift+A&gt;simplify curve and then repeated the process with anew curve and an intermediate step of converting the curve to mesh.
As you can see, the option for simplify curve is unavailable with the new mesh object, as the operator is only usable for curve objects. So, in your situation, either convert back to curve, or add a new object if it is not too tedious of a task.
Here is the documentation for the Simplify Curves add-on.